Structures within the .mu.m-range which are produced photolithographically from photosensitive mixtures, in other words photoresists, play a key role in the production of highly integrated circuits. These types of structures are generally used as overlay masks for a series of process steps, such as ion implantation, doping, wet and dry etching, that are run through when these structural elements are produced.
While the thermal and mechanical properties of the relief structures as well as the etch resistance can be influenced through the selection of the resin base, that is of the polymer of the resist, the photo-configuring capability is predominantly determined by the photoactive component. Photoactive component and polymer, however, must go well together so that the photosensitive mixture does not separate out and the exposed resist is able to be developed. Moreover, the photoactive component must fulfill a series of requirements, such as good solubility, storage stability, ability to be developed with non-toxic, preferably aqueous, i.e. ecologically safe, developing agents, and high sensitivity, that is a short exposure time, which is associated with a bleaching of the photoactive component. In addition, the adherence of the polymer to the substrate must not be adversely affected and an exact transfer of the pattern must be ensured, which in turn is associated with good resolution.
Either negative resists or positive resists can be used to produce fine structures, (c.f. for example: "Angew. Chem."--Applied Chemistry, vol 94 (1982), pp 471 to 485). Negative resists, where the structuring principle is generally based on a cross-linkage of the exposed parts--which is associated with a lowering of the solubility--have a smaller resolution compared to positive resists as a result of swelling. They are mostly developed with--ecologically and economically unfavorable--organic developing agents. In the case of positive resists, where the solubility is increased through the radiation effect, the difference in solubility between the exposed and unexposed parts is mostly produced as the result of a photochemically induced change of polarity. Through this means--without swelling--an aqueous--alkaline development is made possible.
Positive resists are, for example, systems with o-nitrobenzyl-ester compounds as photoactive components, which when exposed decompose into a polar carboxylic acid and an o-nitrosobenzaldehyde (Applied Chemistry, pp 482). The o-nitrosobenzaldehydes, however, can combine in the resist colored azo compounds and thus counteract a bleaching effect. In systems which work according to the principle of so-called "chemical amplification", protons are produced photolytically. These protons then, for example, saponify phenolic esters to the corresponding phenols. In these systems photoinitiators are used, the so-called Crivello salts, such as triaryl sulfonium salts (c.f. for example: Polym. Eng. Sci., vol. 23 (1983), pp 953 to 956). However, they are very corrosive, so that this stands in the way of any possible application in microelectronics.
The most widely used photoactive component, above all in commercially sold positive resists, is based on sulfonic acid esters of diazo naphthoquinones (Applied Chemistry, pp 481), which combine a good bleaching action with short exposure times. However, these types of systems have a series of disadvantages, such as costly synthesis, that is poor availability, high price, inflammable or explosive characteristics (the latter is true above all of 4-sulfonic acids) and low thermal resistance, that is decomposition; in addition, the thick layers (&gt;30 .mu.m) are not able to be structured.
The object of the invention is to provide a photosensitive mixture consisting of a polymer and a photoactive component which fulfill to a great extent the requirements placed on photoresists and which allow fine structures to be produced.